Add tests for all platforms

[tureba]

Adds testing infrastructure with make installcheck support for Linux, macOS/Darwin, and Windows platforms.

Test Infrastructure:

• sql/smoke_test.sql: Quick sanity checks (1-2s runtime)
• sql/system_stats.sql: Comprehensive test suite (5-10s runtime)
• expected/*.out: Expected output files for regression testing

Test Coverage:
Tests all 10 system_stats functions:

1. pg_sys_os_info - OS information (name, version, hostname, arch, boot time)
2. pg_sys_cpu_info - CPU details (cores, clock speed, cache, architecture)
3. pg_sys_memory_info - Memory stats (total, used, free, swap)
4. pg_sys_disk_info - Disk information (space, inodes, filesystems)
5. pg_sys_load_avg_info - Load averages (1, 5, 15 minute)
6. pg_sys_cpu_usage_info - CPU usage percentages
7. pg_sys_io_analysis_info - I/O statistics (reads, writes, bytes)
8. pg_sys_process_info - Process counts (running, sleeping, zombie)
9. pg_sys_network_info - Network stats (interfaces, TX/RX, errors)
10. pg_sys_cpu_memory_by_process - Per-process CPU/memory usage

Test Categories:

• Existence tests: Functions exist and return data
• NULL handling: Required fields are populated
• Data type validation: Correct PostgreSQL types
• Range validation: Values within expected ranges
• Consistency checks: Relationships between values are correct
• Caching behavior: Multiple calls work correctly
• Platform compatibility: Tests pass on Linux, Darwin, Windows

[AndrewJackson2020]

This seems like a very important PR. Would be nice to get CI as well but that is probably out of scope here.

One thought which may be a bad idea: for linux mock a procfs in git controlled test assets and use that path in a test build instead of /proc so we could deterministically assert the expected values.